Remote powering, wherein the power supply is located at a distance from the load, and connected over standard cabling, requires attention to particular factors. For simplicity, the power supply will be herein denoted a power sourcing equipment (PSE) and the load to be powered will be a called a powered device (PD). Firstly, detection by the PSE that a compatible PD is connected prior to supplying power is typically desirable. Secondly, limitation of the amount of current that may be drawn from the PSE over the cabling is preferred so as to provide protection for the PSE and for the cabling in event of a short circuit, or near short circuit at the PD side.
In the event of remote powering in an automobile, additional issues include load dump protection and the varying input voltage with ripple. The term load dump protection in an automotive environment is generally understood to mean protection of the load against extreme voltage transients in the event of disconnection of the battery while the alternator is sending out a charging current. In such a case the voltage output by the alternator may rise from a nominal voltage of about 14 V to a voltage in excess of 100 V for up to 500 msec.
Automotive power supply systems may exhibit a varying direct current (DC) output voltage, based on load, engine speed and battery condition. The varying output voltage may further exhibit some ripple voltage, which may be exhibit a very wide frequency spectrum, including low sub-bass frequency components and frequencies in the megahertz plus ranges. In the event that only a single wire pair is utilized for both power and data transfer, filtering of the high frequency ripple is necessary to avoid data interference. Additionally, such an active filter will reduce the demands on any DC/DC converter in the system, thus allowing for reduced cost.